Field emission devices made with laser and/or plasma treated carbon nanotube mats, films or inks

ABSTRACT

Field emission devices comprising carbon nanotube mats which have been treated with laser or plasma are provided. Mats are formed from carbon nanotubes, also known as carbon fibrils, which are vermicular carbon deposits having diameters of less than about one micron. The carbon nanotube mats are then subjected to laser or plasma treatment. The treated carbon nanotube mat results in improved field emission performance as either a field emission cathode or as part of a field emission device.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/485,918, filed Jul. 9, 2003. This application is also a continuationin part of PCT/US03/19068 filed Jun. 16, 2003, which claims benefit tothe U.S. Provisional Application No. 60/388,616, filed Jun. 14, 2002.This application is also a continuation in part of U.S. Ser. No.10/171,760, filed Jun. 14, 2002, which claims benefit to the U.S.Provisional Application No. 60/298,193, filed Jun. 14, 2001. Thisapplication is also a continuation in part of U.S. Ser. No. 10/171,773,filed Jun. 14, 2002, which claims benefit to U.S. ProvisionalApplication No. 60/298,228, filed Jun. 14, 2001.

TECHNICAL FIELD

The present invention relates to field emission devices or fieldemission cathodes made from or with carbon nanotube mats, films, or inksthat have been laser or plasma treated.

BACKGROUND

Field Emission Devices

Field emission devices are devices that capitalize on the movement ofelectrons. A typical field emission device includes at least a cathode,emitter tips, and an anode spaced from the cathode. A voltage is appliedbetween the cathode and the anode causing electrons to be emitted fromthe emitter tips. The electrons travel in the direction from the cathodeto the anode. These devices can be used in a variety of applicationsincluding, but not limited to, microwave vacuum tube devices, poweramplifiers, ion guns, high energy accelerators, free electron lasers,and electron microscopes, and in particular, flat panel displays. Flatpanel displays can be used as replacements for conventional cathode raytubes. Thus, they have applications in television and computer monitors.

Conventional emitter tips are made of metal, such as molybdenum, or asemiconductor such as silicon. The problem with metal emitter tips isthat the control voltage required for emission is relatively high, e.g.,around 100 V. Moreover, these emitter tips lack uniformity resulting innon-uniform current density between pixels.

More recently, carbon materials, have been used as emitter tips. Diamondhas negative or low electron affinity on its hydrogen-terminatedsurfaces. Diamond tips, however, have a tendency for graphitization atincreased emission currents, especially at currents about thirty mA/cm².Carbon nanotubes, also known as carbon fibrils, have been the latestadvancement in emitter tip technology. Although much work has been donein the area of carbon nanotubes as emitter tips in field emittingtechnologies, substantial improvement is still needed in at least threeareas. These are reducing the working voltage (specific to theparticular application), reducing the “turn-on” voltage, increasingemission current density, and increasing the number of emission sites.Reducing the “turn-on” voltage (and the working voltage) tends toincrease the ease of electron emission and increase the longevity of theemitter tips. Increasing both the emission current and the number ofemission sites increases the brightness. An increased number of emissionsites will likely result in a more homogeneous emission across a givenarea or volume.

Carbon Nanotubes

Carbon nanotubes (CNTs) are vermicular carbon deposits having diametersof less than about one micron. They exist in a variety of forms, andhave been prepared through the catalytic decomposition of variouscarbon-containing gases at metal surfaces, by high temperature carbonarc processes, where solid carbon is used as the carbon feed stock, andby simultaneous laser vaporization of graphite rods and a transitionmetal. Tennent, U.S. Pat. No. 4,663,230, succeeded in growing smalldiameter nanotubes having cylindrical ordered graphite cores and anordered “as grown” graphitic surface uncontaminated with pyrolyticcarbon. Tennent, describes carbon nanotubes that are free of acontinuous thermal carbon overcoat and have multiple graphitic outerlayers that are substantially parallel to the fibril axis. As such theymay be characterized as having their c-axes, the axes which areperpendicular to the tangents of the curved layers of graphite,substantially perpendicular to their cylindrical axes. They generallyhave diameters no greater than 0.1 micron and length to diameter ratiosof at least five. Such nanotubes having graphitic layers that aresubstantially parallel to the fibril axis and diameters between 3.5 and75 nanometers, are described in Tennent et al., U.S. Pat. No. 5,165,909and Tennent et al, U.S. Pat. No. 5,171,560, both of which are hereinincorporated by reference.

The graphitic planes may also be oriented at an angle to the fibrilaxis. Such structures are often called “fishbone” fibrils or nanotubesbecause of the appearance of the two dimensional projection of theplanes. Such morphologies and methods for their production are discussedin U.S. Pat. No. 4,855,091 to Geus, herein incorporated by reference.Fishbone fibrils are typically 10 to 500 nm in diameter, preferably from50 to 200 nm and have aspect ratios between 10 and 1000.

Macroscopic assemblages and composites consisting of multiwall nanotubeshave been described in Tennent et al, U.S. Pat. No. 5,691,054, hereinincorporated by reference. Such assemblages and composites are composedof randomly oriented carbon fibrils having relatively uniform physicalproperties in at least two dimensions. Such macroscopic assemblages aredifferentiated from “as-made” aggregates by the ability to form them atany desired size. Preferably such aggregates have at least one dimensiongreater than 1 mm and preferably greater than 1 cm. Such assemblages maytake the form of a two dimensionally isentropic mat or felt.

The carbon nanotubes disclosed in U.S. Pat. Nos. 4,663,230, 5,165,909,and 5,171,560, may have diameters that range from about 3.5 nm to 70 nmand lengths greater than 100 times the diameters, an outer region ofmultiple essentially continuous layers of ordered carbon atoms and adistinct inner core region. Furthermore, these multiwall nanotubes aresubstantially free of pyrolytically deposited carbon. All of thesereferences are herein incorporated by reference.

As disclosed in U.S. Pat. No. 5,110,693 and references therein (all ofwhich are herein incorporated by reference), two or more individualcarbon fibrils may form microscopic aggregates of entangled fibrils.Simply for illustrative purposes, one type of microscopic aggregate(“cotton candy or CC”) resembles a spindle or rod of entangled fiberswith a diameter that may range from 5 nm to 20 nm with a length that mayrange from 0.1 μm to 1000 μm. Again for illustrative purposes, anothertype of microscopic aggregate of fibrils (“birds nest, or BN”) can beroughly spherical with a diameter that may range from 0.1 μm to 1000 μm.Larger aggregates of each type (CC and/or BN) or mixtures of each can beformed.

Carbon nanotubes having a single wall comprising a single graphene sheethave been produced. These single wall carbon nanotubes have beendescribed in Bethune et al., U.S. Pat. No. 5,424,054; Guo, et al., Chem.Physics Lett., 243: 1-12 (1995); Thess, et al, Science, 273: 483-487(1996); Journet et al., Nature 388 (1997) 756; Vigolo, et al., Science290 (2000) 1331. They are also described in U.S. patent application Ser.No. 08/687,665, entitled “Ropes of Single-Walled Carbon Nanotubes”herein incorporated by reference. Single wall nanotubes may be preparedby a variety of procedures. These may use a solid phase carbon sourcewhich is vaporized by an arc or by a laser. Alternatively, andpreferably, single wall nanotubes are made catalytically from gas phasecarbon precursors. There are two broad methods of such catalyticsynthesis: so-called aerosol or floating catalyst processes using a gasphase catalyst precursor which is decomposed to catalytic species in thereaction zone and processes using a classical supported catalyst.Aerosol processes may advantageously employ elevated pressures of up to100 atm. Supported catalyst processes operate at ambient pressures andmay even be operated at vacuum. Preferred gas phase carbon sources areCO, CH₄, ethanol and benzene. Preferred temperatures are between 500 and1000° C.

Additional methods of producing single wall nanotubes production havebeen described in PCT Application No. PCT/US99/25702 and PCT ApplicationNo. PCT US98/16071 herein incorporated by reference. Single wallnanotubes are useful in a variety of applications. The tubular structureimparts superior strength, low weight, stability, flexibility, thermalconductivity, large surface area and a host of electronic properties.They can be used as reinforcements in fiber reinforced compositestructures or hybrid composite structures, i.e., composites containingreinforcements such as continuous fibers in addition to single wallnanotubes. The carbon nanotubes may be treated in their as-made form ormay be deposited as a film on a suitable substrate and then treated. Allof these references are herein incorporated by reference. NanotubeDeposition Methodology—disclosed in Electrophoretic Deposition ofNanotubes (from U.S. Patent App'n Pub. 2003/0090190, herein incorporatedby reference).

The Electrophoresis Bath

The electrophoretic deposition of the carbon nanotubes may be conductedin an electrophoresis bath. The bath consists of a chamber to containthe solution of carbon nanotubes and means for immersing two opposingelectrodes separated by some distance with the carbon nanotubes betweenthe opposing electrodes. A DC power supply, external to the bath, isused to apply a voltage between the two electrodes immersed in the bath.The cathode lead is connected to the patterned aluminum substrate andthe anode lead is connected to the other electrode. Tantalum was usedfor the second metal. The voltage applied to the two electrodes can beadjusted to a suitable level or the voltage can be adjusted to obtain asuitable current between the two electrodes. The attachment of carbonnanotubes to the aluminum can be enhanced by a binder. The binders canbe a mixture of Ag paste, carbon nanotubes and ethanol. Or the binderscan be a conductive carbon paste, a conductive metal paste or acarbonizable polymer.

Electrophoretic Deposition of Carbon Nanotubes on the Substrate

A field emitter substrate is loaded into the electrophoresis bath. Aplurality of cathodes are arranged on a glass substrate, and adielectric film is formed with holes over the cathodes. Metal gates withopenings which are located over the holes of the dielectric film areformed to expose the surface of the cathodes. Then, the carbon nanotubesare uniformly deposited onto the obtained substrate, on the surface ofthe cathodes exposed through the holes by electrophoretic deposition atroom temperature.

Post Deposition Heat Treatment

After the deposition of carbon nanotube particles by electrophoresis,low-temperature heating is performed to sustain the deposition of thecarbon nanotubes on the cathodes and ensure easy removal of impuritieswhich are incorporated into the field emitter during the deposition.

Preparation of Nanotube Film on Aluminum Substrate (Example from U.S.Patent App'n Pub. 2003/0090190, herein Incorporated by Reference)

With reference to FIG. 17, a solution is formed that contains 150 mli-propyl alcohol (IPA) and 0.44 grams of acid washed carbon nanotubes.This solution is placed in an electrophoresis bath 5000.

A patterned, aluminum coated glass substrate 5002 serves as oneelectrode in electrophoresis bath 5000. The pattern forms the pixelsize. The smallest feature size can be ca. 1 micron. The aluminum coatedglass 5002 is about 55 mm.×45 mm.×1 mm in its dimensions. The aluminumpattern size is about 9 mm×9 mm. The other electrode, tantalum (Ta)electrode 5004 is also inserted into the electrophoresis bath 5000. Aspacer 5006 separates the aluminum coated glass 5002 from the tantalumelectrode 5004. A DC voltage, for example between 40 to 120 volts, e.g.,100 volts is applied to the electrodes. A current between 1.0 to 5 mA,e.g., 3.8 mA, is observed between the electrodes. The duration of thepreparation time can be between about 30 to about 90 minutes, e.g., 60minutes.

FIG. 18 illustrates an alternative electrophoretic method of creatingthe film according to the method disclosed in UK patent application2,353,138 described below. First, a carbon nanotube suspension iscreated. The carbon nanotube particles can have lengths from about 0.1to about 1000 microns. The suspension can also include a surfactant,e.g. an anionic, ionic, amphoteric or nonionic, or other surfactantknown in the art. Examples of suitable surfactants include octoxynol,bis(1-ethylhexyl)sodium sulfosuccinate, and nitrates of Mg(OH)₂, Al(OH)₃and La(OH)₃.

The suspension is then subjected to an electric field to charge thecarbon nanotube particles. The intensity of the electric field and thetime for which the electric field is applied define the thickness of thecarbon nanotube layer. Greater intensity and longer time yield thickerlayers.

With reference to FIG. 18, the field emitter substrate 6030 is loadedinto the electrophoresis bath 6000 containing a carbon nanotubesuspension 6010. An electrode plate 6020 is also installed in theelectrophoresis bath 6000 spaced apart from the field emitter substrate6030. The cathode of a DC power supply 6040, which is installed outsideof the electrophoresis bath 6000, is connected to the other cathodes ofthe field emitter substrate 6030 and the anode of the DC power supply6040 is connected to the electrode plate 6020. Then, a bias voltage ofabout 1 to-about 1000 volts is applied from the DC power supply 6040between the electrode plate 6020 and the cathodes of the field emittersubstrate 6030.

As a positive voltage of the DC power supply 6040 is applied to theelectrode plate 6020, carbon nanotube particles charged by positive ionsin the carbon nanotube suspension 6010 migrate to and are attached tothe exposed cathodes of the field emitter substrate 6030, which resultsin the formation of a carbon nanotube film in the pattern of the exposedcathodes.

The height of the printed carbon nanotube film, also known as the ink,coating, or paste, may be less than 10 microns and the space whichisolates carbon nanotube cathodes from the indium tin oxide anode withindium tin oxide and phosphor is about 125 microns.

The electrophoresis process can be applied to both diodes and triodes.For applications to a diode, an electric field having opposite chargesto those on the surfaces of the carbon nanotube particles is applied toexposed electrode surface of a field emitter substrate for selectivedeposition of carbon nanotube particles thereon. For application to atriode having gates, a weak positive electric field is applied to thegates while a positive electric field is applied to the electrodes ofthe field emitter substrate, which avoids deposition of carbon nanotubeparticles on the gates. In particular, the electrode plate is connectedto the anode of the DC power supply and the cathodes of the fieldemitter substrate are connected to the cathode of the DC power supply.As a positive potential is applied to the gates, the gates repelpositive ions in the carbon nanotube suspension at the surface, whilethe exposed cathodes of the field emitter substrate, which are connectedto the cathode of the DC power supply, pull positive ions of thesuspension through the holes. As a result, the carbon nanotubes aredeposited only on the entire exposed surface of the cathodes, not on thegates of the field emitter substrate. At this time, carbon nanotubeparticles are attracted to the field emitter substrate and are orientedsubstantially horizontal, or substantially parallel to the substrate,which allows the carbon-nanotube particles to smoothly migrate throughthe holes to the cathodes, and thus the carbon nanotubes can bedeposited.

The film can also be prepared similarly to the carbon ink disclosed inEuropean Patent Application EP 1 020 888 A1—Carbon ink,electron-emitting element, method for manufacturing andelectron-emitting element and image display device.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a field emission device is provided.The device has a cathode and an anode. The cathode comprises a carbonnanotube mat wherein the carbon nanotube mat is produced from a filtercake formed by filtering a plurality of nanotubes from a liquidsuspension. The mat may have a top surface and an opposing bottomsurface. The bottom surface corresponds to a filter cake surfacedisposed adjacent to a filter during forming of the mat. The top surfacemay act as an emitting surface of the cathode.

The plurality of nanotubes may have a diameter less than about onemicron.

The plurality of nanotubes may have a morphology resembling a fishbone.The plurality of nanotubes may be single wall or multiwall. Thenanotubes may be oxidized; they may be crosslinked. The filter cake mayhave been formed in the presence of a binder. That binder may, in anembodiment, be a solvent soluble fluoropolymer. It may be PVDF.

Field emission cathodes are provided which comprise a carbon nanotubemat produced from a filter cake formed by filtering a plurality ofnanotubes from a liquid suspension.

A method of treating a field emission cathode comprising nanotubes toimprove turn-on voltage is also provided. The method includesirradiating the cathode with appropriate wavelength radiation forsufficient time and intensity. The radiation may be in the ultravioletrange. In irradiating, the cathode may be exposed to a continuous orpulsed laser. The radiation may have a wavelength of less thanapproximately 349 nm. The radiation may have an energy density greaterthan about 10.3 mJ/cm². Irradiating may be performed in air or may beperformed in a partial pressure of oxygen of at least one torr. Thecathode may be comprised of a carbon nanotube mat. Field emissioncathodes irradiated in this manner are also provided.

Methods of treating a field emission cathode comprising nanotubes toimprove cathode emission current density are provided as furtherembodiments. Methods of treating a field emission cathode comprisingnanotubes to increase the number of emission sites and the uniformity ofemission across the cathode are also provided. These methods include UVirradiation and exposure to low temperature plasma.

A method of orienting nanotubes within a structure comprisingirradiating the structure for a sufficient time and intensity isprovided. A method of orienting nanotubes within a structure comprisingexposing the structure to a low temperature plasma under appropriateconditions is provided.

In another embodiment, the field emission device has a plurality ofnanotubes substantially cylindrical having one or more graphitic layersconcentric with their cylindrical axes, the nanotubes beingsubstantially free of pyrolytically deposited carbon overcoat, having asubstantially uniform diameter between 0.4 nm and 100 nm and having alength to diameter ratio greater than 5 is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the electron emission behavior of electrophoreticallydeposited carbon nanotubes, screen printed carbon nanotubes and carbonnanotube mats in the form of plots of current density as a function ofthe electric field.

FIG. 2 is a series of photographs of electron emission patterns ofelectrophoretically deposited carbon nanotubes, screen printed carbonnanotubes and carbon nanotube mats.

FIG. 3 is a comparative plot of emission current density vs. electricfield (I-V characteristics) for screen printed carbon nanotube cathodeswith and without argon plasma treatment in accordance with anembodiment.

FIG. 4 illustrates a comparison of emission patterns showing an increasein both the number of emission sites and in emission current densityachieved through argon plasma treatment in accordance with anembodiment.

FIG. 5 is a comparison between SEM micrographs of the morphology ofcarbon nanotubes before and after plasma treatment indicated a change inorientation in accordance with a further embodiment.

FIG. 6 is a comparative plot of emission current density vs. electricfield (I-V characteristics) for top and bottom surfaces of CNT matcathodes made with various binders in accordance with an embodiment.

FIG. 7 is a comparative plot of emission current density vs. electricfield (I-V characteristics) for top and bottom surfaces of a CNT matcathode made with a binder before and after irradiation in accordancewith another embodiment.

FIG. 8 illustrates a comparison of emission patterns showing an increasein both the number of emission sites and in emission current densityachieved in CNT mat cathodes after UV laser irradiation in accordancewith a further embodiment.

FIG. 9 is a comparative plot of emission current density vs. electricfield (I-V characteristics) for screen printed CNT cathodes before andafter being exposed to different levels of irradiation in accordancewith another embodiment.

FIG. 10 illustrates a comparison of emission patterns showing anincrease in both the number of emission sites and in emission currentdensity achieved in CNT screen printed cathodes after UV laserirradiation in accordance with yet another embodiment.

FIG. 11 is a comparison between SEM micrographs of the morphology ofcarbon nanotubes before and after laser irradiation treatment indicateda change in orientation in accordance with a further embodiment.

FIG. 12 is a comparison of emission current density obtained afterirradiation with different wavelengths in accordance with an embodiment.

FIG. 13 is a comparison of emission current density obtained afterirradiation with different wavelengths in different irradiationatmospheres (air and vacuum) in accordance with an embodiment.

FIG. 14 shows a comparison of emission current density obtained withscreen print and top and bottom surfaces of mat CNT cathodes before andafter irradiation in accordance with yet another embodiment.

FIG. 15 is a comparison between SEM micrographs of the morphology ofcarbon nanotubes on the top and the bottom surfaces of a CNT mat afterlaser irradiation treatment indicated a change in orientation inaccordance with a further embodiment.

FIG. 16 illustrates a comparison of emission patterns showing anincrease in both the number of emission sites and in emission currentdensity achieved in CNT mat cathodes after UV laser irradiation inaccordance with yet another embodiment.

FIG. 17 illustrates an electrophoresis bath used to fabricate a carbonnanotube film (electrode).

FIG. 18 illustrates another electrophoresis bath used to fabricate acarbon nanotube film (electrode).

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

All referenced patents, patent applications, and publications in thespecification, including the appended bibliography are hereinincorporated by reference herein.

Definitions

“Aggregate” refers to a microscopic particulate structures of nanotubes.

“Assemblage” refers to nanotube structures having relatively orsubstantially uniform physical properties along at least one dimensionalaxis and desirably having relatively or substantially uniform physicalproperties in one or more planes within the assemblage, i.e. havingisotropic physical properties in that plane. The assemblage can compriseuniformly dispersed individual interconnected nanotubes or a mass ofconnected aggregates of nanotubes. In other embodiments, the entireassemblage is relatively or substantially isotropic with respect to oneor more of its physical properties.

“Carbon fibril-based ink” refers to an electroconductive liquidcomposite in which the electroconductive filler is carbon fibrils.

“Graphenic” carbon is a form of carbon whose carbon atoms are eachlinked to three other carbon atoms in an essentially planar layerforming hexagonal fused rings. The layers are platelets having only afew rings in their diameter or ribbons having many rings in their lengthbut only a few rings in their width.

“Graphenic analogue” refers to a structure which is incorporated in agraphenic surface.

“Graphitic” carbon consists of layers which are essentially parallel toone another and no more than 3.6 angstroms apart.

“Low temperature plasma” refers to a gaseous system sufficiently ionizedto be electrically conducting but still electrically neutral, whereinthe electrons are at a higher temperature than the molecules. SeeBaddour, R. F. and Timmins, R. S. eds, The Application of Plasmas toChemical Processing, MIT Press, Cambridge Mass. 1967

“Nanotube”, “nanofiber” and “fibril” and “CNT” are used interchangeably.Each refers to an elongated hollow carbon structure having a diameterless than 1 micron. The term “nanotube” also includes “bucky tubes” andgraphitic nanofibers in which the graphene planes are oriented inherringbone or fishbone pattern.

The terms “emitter tips” and “emitters” are interchangeable. The use ofthe word “tip” is not meant to limit the emission of the electrons onlyto the tips of the carbon nanotubes. The electrons can be emitted fromany part of the carbon nanotubes.

Manufacturing Methods

In addition to electrophoresis (described in background art sectionabove), other processes such as screen printing can be used for creatingthe patterns used to make field emission devices. A screen printingprocess was previously disclosed in U.S. Pat. No. 6,239,547. In additionto screen printing, the carbon nanotubes can be applied to a substrateby ink jet printing. Ink printing is accomplished with carbon nanotubebased liquid media or inks in which the fibrils are nearlyindividualized. Inks typically contain a carrier liquid, carbonnanotubes, and usually also a polymeric binder. Useful binders includeVAGH, VAGF, cellulose acetate butyrate, ethyl cellulose, crosslinkablepolymers and acrylate polymers. These may be present in the range of 1to 7 wt % of the ink. The liquid vehicle may be a polar organic solvent,preferably boiling between 150° C. and 200° C.

Inks may be dried (i.e., evaporate the carrier liquid) to create apatterned coating. Inks are more fully described in PCT/US03/19068incorporated herein by reference. Depending on the printing processcontemplated, inks may have a viscosity between 1 and 50,000 cps. Usefulcarbon nanotube loadings are from 0.5 to 2.5 wt %.

CNT Mats

The carbon nanotubes can also be deposited in the form of a mat. Suchporous mats, having densities between 0.10 and 0.40 gm/cc and surfaceareas greater than 100 sq. m/gm, are conveniently formed by filtrationof suspensions of nanotubes. Methodology is more fully disclosed in U.S.Pat. Nos. 6,099,965 and 6,031,711, both of which are herein incorporatedby reference. If the nanotubes are supplied in the form of aggregates,it is not necessary to fully individualize the nanotubes before making amat. As a simple example, a suspension of nanotubes was preparedcontaining about 0.5% nanotubes in water using a Waring blender. Aftersubsequent dilution to 0.1%, the nanotubes were further dispersed with aprobe type sonifier. The dispersion was then vacuum filtered to form amat, and then oven dried. This results in a filter cake, having a topand bottom surface. Filter material which initially adheres to thebottom surface is peeled away when a mat is successfully made. Oxidizednanotubes are particularly easily dispersed in and then filtered fromaqueous media.

The mats may be subjected to a rigidization or cross linking step asdiscussed in the aforecited patents. Oxidized fibril mats can berigidized by heat treatment in air at up to 300° C. Alternatively, themats may be rigidized by heat treatment in an oxygen free atmosphere atup to 600° C. CNT mat cathodes have uniform emission sites at relativelylow applied field and may obtain a current density of more than 10mA/cm². A comparison of the electron emission behavior ofelectrophoretically deposited carbon nanotubes, screen printed carbonnanotubes and carbon nanotube mats in the form of plots of currentdensity as a function of the electric field is displayed in FIG. 1. Afurther comparison of the electron emission patterns ofelectrophoretically deposited carbon nanotubes, screen printed carbonnanotubes and carbon nanotube mats is displayed in FIG. 2.

CNT mats cathodes may also be produced utilizing various types ofbinders. Useful binders include cellulose, carbohydrates, polyethylene,polystyrene, nylon, polyurethane, polyester, polyamides, phenolic resinsand any other binder than on pyrolysis yields carbon. Pyrolysistemperature depends on the binder used, but can be up to 300° C. in airor up to 900° C. in an oxygen free environment. It is not necessary,however, to pyrolyze the binder. Polymeric binders that can be dissolvedin a solvent containing a fibril suspension and then precipitated byaddition of a non-solvent for that polymer can be used to form mats bysubsequent filtration. PVDF is an example of such a polymer.

Plasma Treatment

In a preferred embodiment, the carbon nanotube or the carbon nanotubemats are subjected to plasma treatment. In yet another preferredembodiment, the screen printed inks containing carbon nanotubes aresubjected to plasma treatment. Alternatively, the field emission cathodeor field emission device itself is subjected to plasma treatment. Plasmatreatment results in improved field emission performance for the carbonnanotube mat or ink, and consequently leads to a better field emissioncathode or field emission device.

Plasma treatment is carried out in order to alter the surfacecharacteristics of the carbon fibrils, fibril structures and/or thematrix, which come in contact with the plasma during treatment; by thismeans the fibril composite treated can be functionalized or otherwisealtered as desired. Once equipped with the teaching herein, one ofordinary skill in the art will be able to adapt and utilize well-knownplasma treatment technology to the treatment of such compositematerials. Thus, the treatment can be carried out in a suitable reactionvessel at suitable pressures and other conditions and for suitableduration, to generate the plasma, contact it with the compositematerial, and effect the desired kind and degree of modification.Plasmas such as those based on oxygen, hydrogen, ammonia, helium, orother chemically active or inert gases can be utilized.

Examples of other gases used to generate plasmas include, argon, water,nitrogen, ethylene, carbon tetrafluoride, sulfurhexafluoride,perfluoroethylene, fluoroform, difluoro-dicholoromethane,bromo-trifluoromethane, chlorotrifluoromethane, and the like. Plasmasmay be generated from a single gas or a mixture of two or more gases. Itmay be advantageous to expose a composite material to more than one typeof plasma. It may also be advantageous to expose a composite material toa plasma multiple times in succession; the conditions used to generatethe plasma, the duration of such successive treatments and the durationof time between such successive treatments can also be varied toaccomplish certain alterations in the material. It is also possible totreat the composite material, e.g., coat the material with a substance,wash the surface of the material, etc., between successive treatments.

Plasma treatment of a composite material may effect several changes. Forexample, a composite material comprising a polymer and a plurality ofcarbon fibrils dispersed therein can be exposed to plasma. Exposure toplasma may etch the polymer and expose carbon fibrils at the surface ofthe composite, thus increasing the surface area of exposed carbonfibrils, e.g., so that the surface area of the exposed fibrils isgreater than the geometric surface area of the composite. Etching of thepolymer may also free nanotube ends or segments that had beenconstrained by the polymer allowing them to move or reorient. Exposureto plasma may introduce chemical functional groups on the fibrils or thepolymer. Treatment can be carried out on individual fibrils as well ason fibril structures such as aggregates, mats, hard porous fibrilstructures, and even previously functionalized fibrils or fibrilstructures. Surface modification of fibrils can be accomplished by awide variety of plasmas, including those based on F₂, O₂, NH₃, He, N₂and H₂, other chemically active or inert gases, other combinations ofone or more reactive and one or more inert gases or gases capable ofplasma-induced polymerization such as methane, ethane or acetylene.Moreover, plasma treatment accomplishes this surface modification in a“dry” process as compared to conventional “wet” chemical techniquesinvolving solutions, washing, evaporation, etc. For instance, it may bepossible to conduct plasma treatment on fibrils dispersed in a gaseousenvironment.

Once equipped with the teachings herein, one of ordinary skill in theart will be able to practice the invention utilizing well-known plasmatechnology. The type of plasma used and length of time plasma iscontacted with fibrils will vary depending upon the result sought. Forinstance, if oxidation of the fibrils' surface is sought, an O₂ plasmawould be used, whereas an ammonia plasma would be employed to introducenitrogen-containing functional groups into fibril surfaces. Once inpossession of the teachings herein, one skilled in the art would be ableto select treatment times to effect the degree ofalteration/functionalization desired.

More specifically, fibrils or fibril structures are plasma treated byplacing the fibrils into a reaction vessel capable of containingplasmas. A plasma can, for instance, be generated by (1) lowering thepressure of the selected gas or gaseous mixture within the vessel to,for instance, 100-500 mTorr, and (2) exposing the low-pressure gas to aradio frequency which causes the plasma to form. Upon generation, theplasma is allowed to remain in contact with the fibrils or fibrilstructures for a predetermined period of time, typically in the range ofapproximately 10 minutes more or less depending on, for instance, samplesize, reactor geometry, reactor power and/or plasma type, resulting infunctionalized or otherwise surface-modified fibrils or fibrilstructures. Surface modifications can include preparation for subsequentfunctionalization.

Treatment of a carbon fibril or carbon fibril structure as indicatedabove results in a product having a modified surface and thus alteredsurface characteristics which are highly advantageous.

Laser Treatment

In a preferred embodiment, the carbon nanotube or the carbon nanotubemats are subjected to laser treatment. In yet another preferredembodiment, the screen printed inks containing carbon nanotubes aresubjected to laser treatment. Laser treatment results in improved fieldemission performance for the carbon nanotube mat or ink, andconsequently leads to a better field emission cathode or field emissiondevice.

With laser treatment, the carbon nanotubes, carbon nanotube mats orcarbon nanotube inks are irradiate with laser (i.e., UV, IR etc) for aperiod of time. Alternatively, the field emission cathode or fieldemission device may also be irradiated with laser.

EXAMPLES

The following examples illustrate the various embodiments of theinvention.

Example 1 Mat with PVDF Binder

Good field emission characteristics were obtained with a CNT mat with aPVDF binder. To prepare the mat, 0.04 grams of PVDF (Kynar 741) wasdissolved in 150 milliliters of acetone. CC type carbon nanotubes, 0.16grams, were blended into the PVDF/acetone solution in a Waring blender.When the suspension appeared uniform, DI water was added, causing thePVDF to precipitate. The CC type carbon nanotubes were entrapped withinthe precipitated PVDF. The precipitate was washed with water, andfiltered onto a Nylon membrane to form a thin mat. The mat was marked sothat the top (air surface) and bottom (Nylon membrane surface) could beidentified. The mat was dried in a low temperature oven (80° C.) andlabeled 296-29-3.

Sections of the CNT mat 296-29-3 were cut and pasted onto the surface ofan aluminum film/glass substrate using silver paste. The I-Vcharacteristics of the CNT mat (both top and bottom surfaces) weremeasured. Further, UV laser irradiation was carried out in air toimprove emission characteristics (see discussion of UV laser irradiationtreatment below). The UV laser emitted a wavelength of 266 nm, had apulse-width of 5 ns, an irradiation energy density of 20.3 mJ/cm², and arepetition frequency of 10 Hz. The laser spot was moved with an overlaprate of 25% within the cathode area after each of irradiation time. Theirradiation was performed for 60 seconds for each spot of the irradiatedarray on the surface of CNT mat.

Example 2 Mat with Surfactant Binder

A stable dispersion of hydrophobic carbon nanotubes can be created withthe use of surface active agents like surfactants and dispersing aids.Mats can then be made using the dispersions. 0.55 grams of SurfynolCT324 (Air Products) was dissolved in 200 mls of DI water. 0.15 grams ofCC-type carbon nanotubes were added and dispersed using a probesonicator (Branson). The dispersed material was filtered onto a Nylonmembrane (0.45 micron pore size) and air dried. When dry the mat couldbe separated from the Nylon membrane. The mat was marked so that the top(air side) and bottom (Nylon membrane side) could be identified. Thismat was labeled 296-29-1.

Alternatively, the mat could be washed to remove any loosely boundSurfynol. 0.60 grams of Surfynol CT324 (Air Products) was dissolved in200 mls of DI water. 0.15 grams of CC-type carbon nanotubes were addedand dispersed using a probe sonicator (Branson). The dispersed materialwas filtered onto a Nylon membrane (0.45 micron pore size) and washedwith methanol by using the vacuum apparatus to pull the methanol throughthe mat. The washed mat was then air dried. When dry the mat could beseparated from the Nylon membrane. The mat was marked so that the top(air side) and bottom (Nylon membrane side) could be identified. Thismat was labeled 296-29-2.

Field emission measurements for samples described in Examples 1 and 2without laser irradiation are shown in FIG. 6. A comparison of fieldemission results for sample 296-29-2 before and after as well as top andbottom surface comparison is shown in FIG. 7. FIG. 8 is a series ofcomparative photographs of electron emission patterns from sample296-29-2 before and after irradiation. FIG. 6 reveals dramatic reductionin turn-on voltage for the top surfaces of samples 296-29-1 and 296-29-3when compared with the top surface of 296-29-2. The results in FIG. 6also show the major improvement in I-V character when top mat surfacesare used as the cathode as opposed to bottom surfaces. FIG. 7illustrates that after 296-29-2, was irradiated, its emissioncharacteristics dramatically improved, almost to the levels of the othertwo samples. These plots are done on logarithmic scale. Thephotomicrographs (FIG. 8) comparing the top surface of 296-29-2 beforean after laser irradiation illustrate how samples appear with an orderof magnitude difference in overall current density at the same workingelectric voltage.

Modification of Carbon Nanotube Films

The carbon nanotubes, or film, may be modified by chemical or mechanicaltreatment. The surface may be treated to introduce functional groups.Techniques that may be used include exposing the carbon nanotubes toelectromagnetic radiation, ionizing radiation, plasmas or chemicalreagents such as oxidizing agents, electrophiles, nucleophiles, reducingagents, strong acids, and strong bases and/or combinations thereof. Ofparticular interest are UV laser irradiation treatment and plasmatreatment.

UV Laser Irradiation Treatment of Nanotube Films

Irradiation treatment is carried out in order to alter the surfacecharacteristics of the carbon fibrils, fibril structures and/or thematrix within which the nanotubes are contained. Numerous experimentshave been performed utilizing UV radiation to enhance cathodeperformance. Initial studies were performed on screen printed CNTcathodes; more recent results have been obtained on CNT mats.

Screen Print

CNT, catalytic grown from hydrocarbon in a gas phase, werescreen-printed on an ITO (indium tin oxide)/glass substrate using aconventional organic binder and baked at 350-450° C. for 30 min in air.The CNT cathode area was 8×8 mm². A diode structure with a spacer of 150μm was used to measure the emission current. The electron emissionpattern was observed through a phosphor screen on the ITO/glasssubstrate, which acts as the anode (anode area: 5×5 mm²) in the diodestructure. The spacer between the anode and the cathode is so thin thatthe electron emission area would be the same size as the anode size.Electric fields shown in the data were defined as an applied anode biasdivided by the spacer thickness minus phosphor/CNT thickness, and theemission current densities were calculated as emission current dividedby the anode area. UV irradiation with wavelengths of 349 and 266 nmfrom a Q-switched tunable Nd:YAG (neodymium: yttrium-aluminum-garnet)laser were used to irradiate CNT cathodes. The repetition frequency ofthe tunable UV laser was 10 Hz with a pulse duration of 5 ns. Laserenergy densities of 20.3, 10.2 and 2.25 mJ/cm² were adjusted by changingthe laser spot area by 4.9, 9.8 and 44.4 mm² at an average laser energyof 1 mJ. Irradiation time was varied from 10 s to 60 s. The laser spotwas moved with an overlap rate of 25% within the cathode area after eachof irradiation time. Irradiations both in air and in vacuum were made atan energy density of 20.3 mJ/cm² for 60 s to study the influence ofatmosphere on laser irradiation. CNT cathodes were also irradiated with30 keV Ga ion beams or exposed to Ar plasma at various conditions forcomparison of energetic treatments.

FIG. 9 shows the I-V characteristics for the CNT emitters before andafter 266 nm laser irradiation in air. The emission current density atan applied electric field of 5.7 V/μm increased after laser irradiationfrom 0.0027 to 14.45 mA/cm² at a laser energy density of 20.3 mJ/cm²(FIG. 10 illustrates a dramatic comparison of before and after emissionpatterns showing an increase in both the number of emission sites and inemission current density) and from 0.0014 to 0.400 mA/cm² at 10.2mJ/cm². The turn-on electric field decreased from 3.7 to 1.2 V/μm at20.3 mJ/mm² and 2.8 to 1.5 V/μm at 10. 2 mJ/cm². On the other hand, noimprovement was found in the sample irradiated at 2.25 mJ/cm². A maximumcurrent density of 20.15 mA/cm² was observed at 6.2 V/μm operatingvoltage. It is felt that the current density would become much higher ifthe phosphor anode could endure a stronger electron bombardment at ahigher electric field.

FIG. 11 shows the CNT morphology before and after laser irradiation at266 nm with an energy density of 20.3 mJ/cm². CNT bundles just afterscreen-printing and baking are tangled together, while after laserirradiation the CNTs tend to orient themselves with open ends. FIG. 12shows the emission current density as a function of laser energy densityfor specifically 266 and 349 nm laser irradiations. The range of theemission current density before laser irradiation was indicated as a barin the figure. The current density increases by 4 orders of magnitude(about 1 μA/cm² before to 14.45 mA/cm² after laser irradiation.) Muchbetter improvement was observed for 266 nm irradiations than for 349 nmirradiations. The difference in improvement by different wavelengthssuggests, although not to be bound by a particular theory, that thelaser induced reaction is not a simple thermal process, though muchhigher energy of laser photons is necessary for direct bond breaking ofC═C bonds (6.3 eV). It is suggested that the effect induced by UV laserirradiation would be mainly due to the photo-excitation effect such asphoto-decomposition rather than thermal effect. For the CNT cathodeirradiated by 266 nm laser lights, the emission current density seems tobe saturated after 100 shots. On the other hand, as the time increased,the emission current density also increased in the case of 349 nm laserirradiation. This suggests that, in the case of 266 nm laserirradiation, almost all the chemical bonds of C—H, H—O of the organicbinder, remained within the CNT cathodes after baking, were broken bythe photons and/or were oxidized, and the organic binder was decomposedwith less irradiation time. On the other hand, many more laser shots arerequired for the 349 nm irradiation to decompose the residual organicbinder on the surface because of the lower energy of photons. Theimprovements in the emission characteristics would be photo excitationand decomposition effect rather than thermal effect, since thetemperature rise by laser photons at these wavelengths is of almost allthe same level.

The influence of irradiation atmosphere on emission current density (forair vs. vacuum) at a laser energy density of 20.3 mJ/cm² for 60 seconds(600 shots) is shown in FIG. 13. The emission current density increasesby about 3 orders of magnitude when the samples were irradiated by a 266nm laser light in air, whereas the improvement of emission currentdensity with irradiations in vacuum was only slightly observed. Thisindicates the contribution of oxygen during irradiation, i.e.,oxidation. Thus the improvement with 266 nm laser irradiation was muchpronounced than that by 349 nm laser irradiation in this case too.Further investigation on the oxygen pressure dependence of the laserirradiation is necessary for clarify the effect of laser irradiation.

CNT Mat

A CNT mat was pasted on the surface of an aluminum film/glass substrateusing silver paste. Previously, CNT samples were screen-printed on anITO (Indium Tin Oxide)/glass substrate with organic binder. The I-Vcharacteristics of the CNT mat (for top and bottom surfaces) andscreen-printed CNT emitters were tested. Further, UV laser irradiationwas carried out in air to improve emission characteristics [3, 4]. TheUV laser of 266 nm has a pulse-width of 5 ns, an irradiation energydensity of 20.3 mJ/cm², and a repetition frequency of 10 Hz. Theirradiation lasted for 60 seconds for each spot of the irradiated arrayon the surface of CNT emitters.

FIG. 14 shows the emission current density as a function of electricfield for CNT mat and screen-printed CNT emitters before and after UVlaser irradiation in air. With an applied electric field of 3.47 V/μm,the emission current density of the top and bottom surfaces of CNT matare 1.99 and 0.03 mA/cm², and for the screen-printed CNT emitters, noemission was observed with the same electric field. After UV laserirradiation, the emission current density changed to 1.52 and 6.76mA/cm² for the top and bottom surfaces of CNT mat, and the emissioncurrent density of screen-printed CNT emitters increased to 0.33 mA/cm².As shown in the SEM photomicrographs of CNT mat in FIG. 15, themorphologies of the two surfaces are quite different. FIG. 16 shows thatthe emission uniformity was also improved after UV laser irradiation. Toput this data into perspective, it should be noted that the electronemission behavior of CNT mat without any surface treatment is similar tothat of the screen-printed CNT cathodes after laser irradiation.

Example 3 CNT Cathodes

As shown in FIGS. 3, 4, and 5, the emission characteristics from carbonnanotube (CNT) cathodes made by screen printing on a glass substratewere measured after aging in an ultra high vacuum chamber (5.3×10⁻⁸ Pa).CNT cathodes on the glass substrate were exposed to argon (Ar) plasmawith a discharge voltage of 250 V and a vacuum of 40 Pa. The plasmaexposure time was varied in following times (30 s, 1 min, 2 min, 3 min,4 min and 5 min). The emission characteristics from CNT cathodes weremeasured in an ultra high vacuum chamber. FIG. 3 shows I-Vcharacteristics of CNT cathodes before and after plasma treatment for 3min. The emission current increased after an Ar plasma treatment for 3min by three orders of magnitude from 9.0×10⁻⁵ to 0.3 mA/cm² at 4 V/μmfield with a decrease in turn-on voltage from 3.3 V/μm to 1.7 V/μm. FIG.4 shows electron emission patterns at 4.6 V/μm; (a) before plasmatreatment; (b) after plasma treatment for 3 min. This clearly indicatesan increase in the number and the strength of the emission sites afterplasma treatment. These results indicate that the emissioncharacteristics were significantly improved by plasma treatment. FIG. 5shows the CNT images using scanning electron microscopy (SEM) before andafter plasma treatment. CNT bundles that are tangled together afterscreen printing were somewhat unraveled, leading to a degree oforientation perpendicular to the cathode surface Ar plasma treatment.Not to be bound by a particular theory, it is believed that thisorientation effect observable with the CNT after plasma treatmentcontributes to the enhanced electron emission after plasma treatment.

No experiments have been performed to date investigating the effects ofplasma treatment on nanotube mat cathodes. Recalling the emissionresults shown in FIG. 2 in comparing untreated screen printed electrodeswith untreated mat electrodes, it is anticipated that plasma treated matelectrodes will yield an improved cathode.

Example 4 Carbon Nanotube Ink

A carbon nanotube containing ink (sample 296-47-02) was prepared asfollows. The polymeric binder and liquid vehicle were first prepared bymixing 9.5 grams of VAGH (DOW Hydroxyl-Modified Vinyl Copolymer) with100 grams of y butyrolactone on a hot plate with a stir bar at 60° C.until the binder had completely dissolved. After dissolution of theVAGH, a clear light yellow solution was obtained. 1 gram of Triton-Xsurfactant was added to the solution and agitated to dissolve. 2.0 gramsof dry carbon fibrils were added and the mixture was sonicated with aprobe Branson sonicator at 450 W. The sonication continued until agel-like slurry was obtained. A three roll mill was then used to millthe ink to a uniform, viscous ink. The ink was processed through 4passes through the three roll mill and the ink was finally filteredthrough a 500 mesh stainless steel filter screen.

Example 5 Carbon Nanotube Ink with Plasma Treatment

The emission characteristics from carbon nanotube (CNT) cathodes made byscreen printing a carbon nanotube ink on a glass substrate were measuredafter aging in an ultra high vacuum chamber (5.3×10⁻⁸ Pa). CNT cathodeson the glass substrate were exposed to argon (Ar) plasma with adischarge voltage of 250 V and a vacuum of 40 Pa. The plasma exposuretime was varied in following times (30 s, 1 min, 2 min, 3 min, 4 min and5 min). The emission characteristics from CNT cathodes were measured inan ultra high vacuum chamber. The emission current increased after an Arplasma treatment for 3 min by three orders of magnitude from 9.0×10⁻⁵ to0.3 mA/cm² at 4 V/μm field with a decrease in turn-on voltage from 3.3V/μm to 1.7 V/μm. The electron emission patterns at 4.6 V/μm; before andafter plasma treatment for 3 min. clearly indicate an increase in thenumber and the strength of the emission sites after plasma treatment.Scanning electron microscopy (SEM) images of the cathode surface beforeand after plasma treatment were recorded. These images show that bundlesthat are tangled together after screen printing were somewhat unraveled,leading to a degree of orientation perpendicular to the cathode aftersurface Ar plasma treatment. Not to be bound by a particular theory, itis believed that this orientation effect observable with the CNT afterplasma treatment contributes to the enhanced electron emission afterplasma treatment. These results indicate that the emissioncharacteristics were significantly improved by plasma treatment.

Example 6 Cathode Prepared from Carbon Nanotube Ink Treated with LaserIrradiation

The emission characteristics from carbon nanotube (CNT) cathodes made byscreen printing a carbon nanotube ink on a glass substrate were measuredbefore and after treatment with a UV laser. UV laser light withwavelength of 349 and 266 nm from a tunable laser were used to irradiateCNT samples in air and in a vacuum chamber (pressure: 1×10⁻⁵ pa) for 1minute at an average energy density of 20.3, 10.2 and 2.25 mJ/cm²respectively, corresponding to an elliptic beam spot size of 4.9, 9.8and 44.4 mm². The repetition frequency of the laser is 10 Hz, with apulse duration of 5 ns. A diode structure (anode area: 5×5 mm²) with aspacer of 125 mm was used to measure the emission current. The electronemission pattern was observed through a phosphor screen on the ITOanode. The emission characteristics of CNT samples irradiated in air byboth of the 349 and 266 nm UV laser with an average irradiation energydensity of 20.3 and 10.2 mJ/cm² were drastically improved after laserirradiation. For example, the emission current densities were increasedfrom 8.9 to 259.4 mA/cm², and the turn-on electric field were decreasedfrom 3.6 to 2.9 V/micron.

One skilled in the art will appreciate further features and advantagesof the invention based on the above-described embodiments. Accordingly,the invention is not to be limited by what has been particularly shownand described, except as indicated by the following appended claims.

All of the references listed below are hereby incorporated by reference:

BIBLIOGRAPHY

Use of Carbon Nanotubes in Field Emission Cathodes for Light Sources

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O. Yavas, et al., Laser Cleaning of Field Emitter Arrays for EnhancedElectron Emission, 72 APPLIED PHYSICS LETTERS 2797 (1998)

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1. A field emission device comprising: an anode; and a cathodecomprising a carbon nanotube mat; wherein the carbon nanotube mat isproduced from a filter cake formed by filtering a plurality of nanotubesfrom a liquid suspension.
 2. A device according to claim 1, the mathaving a top surface and an opposing bottom surface, the bottom surfacecorresponding to a filter cake surface disposed adjacent to a filterduring forming; such that the top surface acts as an emitting surface ofthe cathode.
 3. A device according to claim 1, wherein the plurality ofnanotubes have a diameter less than about one micron.
 4. A deviceaccording to claim 1, wherein the plurality of nanotubes have amorphology resembling a fishbone.
 5. A device according to claim 1,wherein the plurality of nanotubes have a single wall.
 6. A deviceaccording to claim 1, wherein the plurality of nanotubes are oxidized.7. A device according to claim 6, wherein the nanotubes are crosslinked.8. A device according to claim 1, wherein the filter cake is formed inthe presence of a binder.
 9. A device according to claim 8, wherein thebinder is a solvent soluble fluoropolymer.
 10. A device according toclaim 9, wherein the binder is PVDF.
 11. A field emission cathodecomprising: a carbon nanotube mat; wherein the carbon nanotube mat isproduced from a filter cake formed by filtering a plurality of nanotubesfrom a liquid suspension.
 12. A cathode according to claim 11, the mathaving a top surface and an opposing bottom surface, the bottom surfacecorresponding to a filter cake surface disposed adjacent to a filterduring forming; such that the top surface acts as an emitting surface ofthe cathode.
 13. A method of treating a field emission cathodecomprising nanotubes, the method comprising irradiating the cathode withappropriate wavelength radiation for sufficient time and intensity. 14.A method according to claim 13 wherein, in irradiating, the radiation isin the ultraviolet range.
 15. A method according to claim 13 wherein, inirradiating, the cathode is exposed to a laser.
 16. A method accordingto claim 15 wherein the laser is pulsed.
 17. A method according to claim15 wherein the radiation has a wavelength of less than approximately 349nm.
 18. A method according to claim 15 wherein the radiation has anenergy density greater than about 10.3 mJ/cm².
 19. A method according toclaim 13 wherein irradiating is performed in air.
 20. A method accordingto claim 13 wherein irradiating is performed in a partial pressure ofoxygen of at least one torr.
 21. A method according to claim 13 whereinthe cathode comprises a carbon nanotubes mat; wherein the carbonnanotubes mat is produced from a filter cake formed by filtering aplurality of nanotubes from a liquid suspension.
 22. A method accordingto claim 21, the mat having a top surface and an opposing bottomsurface, the bottom surface corresponding to a filter cake surfacedisposed adjacent to a filter during forming; such that the top surfaceacts as an emitting surface of the cathode.
 23. A method according toclaim 13, wherein the plurality of nanotubes have a diameter less thanabout one micron.
 24. A method according to claim 13, wherein theplurality of nanotubes have a morphology resembling a fishbone.
 25. Amethod according to claim 13, wherein the plurality of nanotubes have asingle wall.
 26. A method according to claim 13, wherein the pluralityof nanotubes are oxidized.
 27. A method according to claim 13, whereinthe nanotubes are crosslinked.
 28. A method according to claim 21,wherein the filter cake is formed in the presence of a binder.
 29. Amethod according to claim 28, wherein the binder is a solvent solublefluoropolymer.
 30. A method according to claim 29, wherein the binder isPVDF.
 31. A field emission cathode treated according to claim
 13. 32-51.(canceled)
 52. A method of treating a field emission cathode comprisingnanotubes, the method comprising exposing the cathode to a lowtemperature plasma under appropriate conditions.
 53. A method accordingto claim 52 wherein the plasma is a noble gas plasma.
 54. A methodaccording to claim 53 wherein the plasma is an argon plasma.
 55. Amethod according to claim 52 wherein the plasma is a gas mixturecomprising a noble gas.
 56. A method according to claim 52 wherein thecathode comprises a carbon nanotube mat; wherein the carbon nanotube matis produced from a filter cake formed by filtering a plurality ofnanotubes from a liquid suspension.
 57. A method according to claim 56,the mat having a top surface and an opposing bottom surface, the bottomsurface corresponding to a filter cake surface disposed adjacent to afilter during forming; such that the top surface acts as an emittingsurface of the cathode.
 58. A method according to claim 52, wherein theplurality of nanotubes have a diameter less than about one micron.
 59. Amethod according to claim 52, wherein the plurality of nanotubes have amorphology resembling a fishbone.
 60. A method according to claim 52,wherein the plurality of nanotubes have a single wall.
 61. A methodaccording to claim 52, wherein the plurality of nanotubes are oxidized.62. A method according to claim 52, wherein the nanotubes arecrosslinked.
 63. A method according to claim 56, wherein the filter cakeis formed in the presence of a binder.
 64. A method according to claim63, wherein the binder is a solvent soluble fluoropolymer.
 65. A methodaccording to claim 64, wherein the binder is PVDF.
 66. A field emissioncathode treated according to claim
 52. 67. A method of orientingnanotubes within a structure, the method comprising irradiating thestructure for a sufficient time and intensity.
 68. A method of orientingnanotubes within a structure, the method comprising exposing thestructure to a low temperature plasma under appropriate conditions. 69.A device according to claim 1, wherein the plurality of nanotubes aresubstantially cylindrical having one or more graphitic layers concentricwith their cylindrical axes, the nanotubes being substantially free ofpyrolytically deposited carbon overcoat, having a substantially uniformdiameter between 0.4 nm and 100 nm and having a length to diameter ratiogreater than
 5. 70. A method according to claim 13, wherein theplurality of nanotubes are substantially cylindrical having one or moregraphitic layers concentric with their cylindrical axes, the nanotubesbeing substantially free of pyrolytically deposited carbon overcoat,having a substantially uniform diameter between 0.4 nm and 100 nm andhaving a length to diameter ratio greater than
 5. 71. A method accordingto claim 13 wherein the cathode comprises a film; wherein the film isproduced by placing an ink onto a substrate, the ink comprising acarrier liquid and carbon nanotubes.
 72. A method according to claim 71wherein placing the ink is accomplished by a process selected from agroup consisting of screen printing, ink-jet printing and spraypainting.
 73. A field emission cathode treated according to claim 71.74. A method according to claim 52 wherein the plasma is selected from agroup consisting of oxygen, hydrogen, ammonia, helium, argon, water,nitrogen, ethylene, carbon tetrafluoride, sulfur hexafluoride,perfluoroethylene, fluorine, fluoroform, chlorine,difluoro-dichloromethane, bromo-trifluoromethane,chloro-trifluoromethane, bromine and mixtures thereof. 75-82. (canceled)83. A method according to claim 13, wherein irradiating improves turn onvoltage characteristics of the cathode.
 84. A method according to claim13, wherein irradiating improves cathode emission current density.
 85. Amethod according to claim 13, wherein irradiating increases number ofemission sites and uniformity of emission across the cathode.
 86. Amethod according to claim 52, wherein the cathode comprises a film;wherein the film is produced by placing an ink onto a substrate, the inkcomprising a carrier liquid and carbon nanotubes.
 87. A method accordingto claim 86, wherein placing the ink is accomplished by a processselected from the group consisting of screen printing, ink-jet printingand spray painting.
 88. A field emission cathode according to claim 86.